ECHINODEKMATA 



149 



ish has given rise to the phrase radial symmetry. Sym- 

 letry involves a repetition of similar parts. Externally 

 le arms of the starfish radiate from a common center 



ig. 83 B.— Starfish, ventral surface (placed on back, righting itself). (Prom 

 Packard, after Romanes.) 



md except for the perforated plate on the dorsal sur- 

 face the five parts of the body duplicate each other, 

 "nternally the organs are correspondingly repeated. 

 The perforated plate gives the body a rudimentary 

 two-sided or bilateral symmetry, for a line drawn 

 through this plate to the tip of the opposite ray divides 

 the animal into two similar halves. 



Internal Structure. — The alimentary system is a tube 

 open at both ends that passes through the animal dorso- 



